Activated protein C is a potent inhibitor of blood coagulation. Defects in protein C and the proteins that regulate its activity may lead to the development of thrombosis. The long term goal of this project is to determine the molecular basis of the action of protein C and the mechanisms of its regulation. The purpose of this project is to study the mechanisms by which protein S, a cofactor for activated protein C, is regulated. The first aim of the research is to characterize the proteins in plasma and formed elements of blood that interest with protein S. This will involve isolation of these proteins by standard chromatographic methods and determination the relationship between the proteins through sequence analysis and immunochemistry. Secondly, the functional significance of the proteins will be ascertained by determining the concentrations of the components and the affinities between the binding proteins and protein S. In addition the effects of these proteins on the anticoagulant activity of activated protein C will be determined. The third aim of the project is to study the relationship between protein S primary structure and the functional properties of the protein. This will be carried out by studies of peptides found in the protein S sequence, chemical modification experiments and characterization of the functional properties of chemically modified protein S.